Method and installation for the densification of substrates by means of chemical bapour infiltration

ABSTRACT

A method of densifying porous substrates by chemical vapor infiltration comprises loading porous substrates for densification in a loading zone of an enclosure ( 10 ), heating the internal volume of the enclosure, and introducing a reagent gas into the enclosure though an inlet situated at one end of the enclosure. Before coming into contact with substrates ( 20 ) situated in the loading zone, the reagent gas admitted into the enclosure is preheated, at least in part, by passing along a duct ( 30 ) connected to the gas inlet and extending through the loading zone, the duct being raised to the temperature inside the enclosure, and the preheated reagent gas is distributed in the loading zone through one or more openings ( 33 ) formed in the side wall ( 32 ) of the duct, along the duct.

BACKGROUND OF THE INVENTION

[0001] The invention relates to chemical vapor infiltration techniques.The field of application of the invention is densifying poroussubstrates, in particular making composite material parts by densifyingfiber substrates by means of a matrix.

[0002] In conventional manner, a method of densifying substrates bychemical vapor infiltration comprises the steps of loading poroussubstrates to be densified into a loading zone of an enclosure, heatingthe internal volume of the enclosure, introducing a reagent gas in theenclosure through an inlet situated at one end thereof, and preheatingthe reagent gas after it has entered into the enclosure and before itcomes into contact with the parts situated in the loading zone.

[0003] The temperature and the pressure that exist inside the enclosureare selected so as to enable the reagent gas to diffuse into the poresof the substrates and deposit therein the material for constituting thematrix, either by one or more components of the reagent gas decomposing,or else by a plurality of components reacting together.

[0004] The reagent gas is conventionally preheated by passing the gasthrough a preheater zone situated inside the enclosure and into whichthe reagent gas inlet opens out. A conventional preheater zone comprisesa plurality of perforated plates disposed one above the other and raisedto the temperature inside the enclosure.

[0005] The purpose of preheating the reagent gas is to ensure that whenit enters into the loading zone it is at a temperature that is as closeas possible to the temperature required for forming the desired matrix.When the reaction temperature is typically about 1000° C. in order toform a matrix of pyrolytic carbon or of ceramic, having the reagent gasat a temperature that is only a few tens of ° C. below the desiredtemperature can have a significant effect on the rate of densificationand on the microstructure of the deposited matrix material.

[0006] This has been observed particularly in the case of densifyingsubstrates disposed in stacks, in particular substrates of annular shapefor making brake disks out of composite material. Methods andinstallations for densifying annular substrates in stacks are describedin documents U.S. Pat. No. 5,904,957 and EP 0 792 385. The reagent gascoming from the preheater zone is admitted into the internal volumes ofthe stacks which are made up of superposed annular substrates and whichextend vertically in the loading zone above the preheater zone, with thereagent gas inlet being situated at the bottom of the enclosure. Adensification gradient is observed between the substrates situated atthe bottoms of the stacks and the other substrates, which gradientbecomes greater the more insufficient the preheating of the reagent gas.

[0007] The problem could be solved by increasing the volume of thepreheater zone. However, for a given total enclosure volume, that wouldreduce the space available into which substrates can be loaded.Unfortunately, the processes of densification by chemical vaporinfiltration are lengthy and expensive to implement, so installationsneed to have their loading capacities used to the full.

[0008] In addition, the reagent gas reaching the tops of the stacks hastraveled through them along their full height and has matured, such thatthe substrates situated at the top of the stacks receive a reagent gasof composition that may be different from that of the reagent gas onentering into the loading zone. This also can give rise to densificationcharacteristics that are different.

OBJECT AND SUMMARY OF THE INVENTION

[0009] An object of the invention is to provide a method enabling thedistribution and the preheating of the reagent gas to be improved, andmore generally enabling densification gradients between substratessituated at different locations in the loading zone to be reduced, andto achieve this without decreasing loading capacity, and possibly evenwhile increasing it.

[0010] This object is achieved by a method as defined in theintroduction to the description and in which the reagent gas admittedinto the enclosure is preheated, at least in part, by passing along aduct connected to the gas inlet and extending through the loading zone,the duct being raised to the temperature inside the enclosure, and thepreheated reagent gas is distributed into the loading zone through oneor more openings formed in the side wall of the duct, along the lengththereof.

[0011] Thus, the duct serves both to preheat the reagent gas and todistribute it in the loading zone.

[0012] The reagent gas may be distributed via one or more slotsextending longitudinally through the side wall of the duct.

[0013] In a variant, the reagent gas may be distributed in the enclosurevia a plurality of perforations formed through the side wall of theduct.

[0014] In order to enhance preheating, the reagent gas advantageouslyflows inside the duct while making contact with walls forming heatexchanger surfaces that extend into the inside of the duct.

[0015] When densifying annular substrates placed in the loading zone inat least one vertical stack, the reagent gas admitted into the enclosureis advantageously preheated and distributed by passing along a ductextending vertically inside the stack.

[0016] The reagent gas is then preferably distributed solely viaopenings formed in the side wall of the duct.

[0017] Another object of the invention is to provide an installationenabling the above-defined method to be implemented.

[0018] This object is achieved by an installation comprising anenclosure inside which there is a zone for loading substrates to bedensified, a susceptor defining the enclosure and associated with meansfor heating the enclosure, a reagent gas inlet at one end of theenclosure, and means situated inside the enclosure for preheating thereagent gas, in which installation a duct is connected to the reagentgas inlet inside the enclosure and extends through the loading zone, theduct being provided along its length with lateral openings which openout into the loading zone in order to distribute the reagent gastherein.

[0019] In an embodiment, the openings are in the form of at least onelongitudinal slot. The wall of the tube may then be formed by aplurality of panels leaving longitudinal gaps between one another.

[0020] In another embodiment, the openings are in the form ofperforations distributed along the duct.

[0021] Advantageously, walls are disposed inside the duct. Theseinternal walls can then be in the form of longitudinal panels that leavegaps between one another.

BRIEF DESCRIPTION OF THE DRAWINGS

[0022] The invention will be better understood on reading the followingdescription given by way of non-limiting indication and made withreference to the accompanying drawings, in which:

[0023]FIG. 1 is a diagrammatic elevation view in section showing aninstallation for densification by chemical vapor infiltration in anembodiment of the invention;

[0024]FIG. 2 is a fragmentary cross-section view on a larger scaleshowing more particularly the duct for preheating and distributing thereagent gas in the FIG. 1 installation;

[0025]FIG. 3 is a cross-section view showing a variant embodiment of theduct for preheating and distributing the reagent gas;

[0026]FIG. 4 is a diagrammatic elevation view in section showing anembodiment of a prior art installation for densification by chemicalvapor infiltration;

[0027]FIG. 5 is an elevation view showing another embodiment of a ductfor preheating and distributing the reagent gas;

[0028]FIG. 6 is a cross-section view of the FIG. 5 duct;

[0029]FIG. 7 is a diagrammatic elevation view in section showing aninstallation for densification by chemical vapor infiltrationconstituting another embodiment of the invention; and

[0030]FIGS. 8 and 9 are diagrammatic elevation views in section showingher applications of an installation of the invention.

DETAILED DESCRIPTION OF EMBODIMENTS

[0031]FIG. 1 is a diagram of an enclosure 10 containing a load of poroussubstrates 20. By way of example, the substrates 20 are carbon fiberpreforms or blanks constituted by pre-densified preforms, which preformsor blanks are for use in making brake disks of carbon/carbon (C/C)composite material by being densified with a matrix of pyrolytic carbon.

[0032] The load is in the form of a stack of substrates defining aninside volume 21 formed by the central passages in thevertically-aligned substrates. The stack is carried by a bottom supportplate 11 standing on legs 12 a. It may be made up of a plurality ofsuperposed sections that are separated from one another by one or moreintermediate support plates 13. The plate 11 is provided with an opening11 a which is in axial alignment with the central passages through thesubstrates 20 and with openings 13 a in the intermediate plates 13. Atits top, the stack of substrates is provided with a cover 22 closing theinternal volume 21. The plates 13 are supported by the support plate 11via columns or posts 12 b.

[0033] Each substrate 20 is separated from an adjacent substrate, andwhere appropriate from an adjacent plate 11 or 13 or the cover 22 by oneor more spacers 23 which define gaps 24 (see FIGS. 1 and 2). The spacers23, which are disposed radially for example, are arranged to formpassages that put the internal volume 21 into communication with anexternal volume 25 situated inside the enclosure and outside the stack.

[0034] The passages left between the spacers 23 may be dimensioned insuch a manner as to balance pressures between the volumes 21 and 25, asdescribed in document U.S. Pat. No. 5,904,957. In a variant, they mayconstitute leakage passages providing a flow section that is small so asto allow a pressure gradient to exist between the volumes 21 and 25, asdescribed in French patent application No. 01/03004.

[0035] The enclosure is heated by means of a susceptor 14 which definesthe sides of the enclosure. By way of example, the heater plate isconstituted by an inductor inductively coupled with an induction coil15. The coil 15 surrounds the enclosure and is separated from thesusceptor 14 by a wall 16 that provides thermal insulation. In avariant, the susceptor may be heated by means of electrical resistancesthermally coupled therewith.

[0036] A reagent gas containing one or more constituents that areprecursors of carbon is introduced into the enclosure through an opening17 a formed in the bottom 17 of the enclosure. The precursors aregaseous hydrocarbons, typically methane, propane, or a mixture thereof.In the gap between the bottom 17 and the plate 12, the reagent gas ischanneled by a cylindrical wall 18 interconnecting the openings 17 a and11 a.

[0037] A vertical tubular duct 30 has its bottom end connected to theopening 11 a and extends vertically inside the volume 21 to theimmediate vicinity of the top of the stack of substrates. At its topend, the duct 30 is closed by a cover 31. The duct 30 may be made up ofa plurality of sections connected end to end so as to enable it to bebuilt up in modular manner.

[0038] In the example shown in FIGS. 1 and 2, the duct 30 has its sidewall 32 provided with a plurality of openings 33 in the form ofperforations which are distributed both along the length of the duct 30and around the axis thereof.

[0039] Thus, the reagent gas admitted into the enclosure is distributedinto the internal volume 21 by passing through the openings 33 in theduct 30 and passes from the volume 21 to the volume 25 by diffusingthrough the substrates 20 and passing through the passages left betweenthe spacers 23. The residual gas is extracted from the enclosure 10 viaan opening 19 a formed through the cover 19 of the enclosure andconnected to suction means (not shown).

[0040] The duct 30 serves not only to distribute the reagent gas overthe full height of the stack, but also to preheat this gas, the duct 30being raised to the temperature that exists inside the enclosure.

[0041] In order to improve preheating, internal heat exchanger walls maybe disposed inside the duct 30. In the embodiment of FIG. 3, theseinside walls are in the form of longitudinal panels 35 distributedaround the axis of the duct and leaving gaps 36 between one another.

[0042] The duct 30, the cover 31, and any internal walls 35 are made ofgraphite, for example. Other materials could be used, for example a C/Ccomposite material. The walls 14, 17, 19 of the enclosure 10 areadvantageously made of graphite. The plates 11, 13, the cover 22, thespacers 23, and the wall 18 are made, for example, out of graphite orout of C/C composite material.

[0043] In comparison with a prior art installation having a preheaterzone 1 between the reagent gas inlet and the plate 11 on which the stackstands (see FIG. 4), the installation of FIGS. 1 and 2 does not have apreheater zone, thereby providing significantly increased loadingcapacity. The loading zone of the enclosure 10 which extends above theplate 11 is greater than the loading zone in the installation of FIG. 4,with the preheater zone and the perforated plates 2 situated one aboveanother occupying a relatively large amount of space in thatinstallation.

[0044] Nevertheless, it should be observed that it is possible for apreheater zone to be present in the context of the invention, which zonecan be smaller than those of prior art installations.

[0045] The diameter of the duct 30 must be large enough to be capable ofproviding a large area for heat exchange, while nevertheless beingspaced apart from the stack of substrates 20.

[0046]FIGS. 5 and 6 show a variant embodiment of a duct 40 forpreheating and distributing reagent gas that can take the place of theduct 30 in the installation of FIGS. 1 and 2.

[0047] The side wall 42 of the duct 40 has openings 43 in the form oflongitudinal slots that extend over the entire length of the duct, theduct being closed by a cover 41 at its top end. In the example shown,the slots 43 are rectilinear and they are regularly distributed aroundthe axis of the duct 40.

[0048] The slots 43 are formed by gaps between longitudinal panels 44that make up the side wall 42 of the duct 40. Additional internal wallsfor heat exchange purposes are disposed inside the duct 40. As in theembodiment of FIG. 3, these internal walls are in the form oflongitudinal panels 45 distributed around the axis of the duct andleaving gaps 46 between one another. The panels 44 and 45 are disposedin a staggered configuration around the axis of the duct 40 so that eachgap 46 opens out facing a panel 44 between two slots 43.

[0049] Naturally, the slots could follow paths other than rectilinearpaths, for example they could follow helical paths from the bottom tothe top of the duct.

[0050] In general, it is possible to give any desired shape to theopenings formed in the side wall of the duct, for example oblong shapesor elongate openings extending axially, circumferentially, or obliquely.

[0051] In the embodiment of FIGS. 1 and 2, a single stack of substrates20 is shown. In a variant, a plurality of stacks of substrates could beplaced side by side inside the enclosure. In which case, a respectiveduct for preheating and distributing reagent gas is placed inside eachstack and is connected to a common inlet for the reagent gas, orpreferably to a particular inlet in alignment with the duct.

[0052] It should also be observed that the flow direction of the reagentgas may be reversed, with a gas inlet being formed through the cover ofthe enclosure and an outlet formed in the bottom which is spaced apartfrom the plate supporting the stack, with the central passage of thestack then being closed at its bottom end.

[0053] As shown in FIG. 7, and in the same manner as shown in FIG. 1,the stack of annular substrates 120 is received in an enclosure 110defined laterally by a susceptor 114 inductively coupled with aninduction coil 115, there being insulation 116 disposed between them.The stack of substrates 120 is formed by a plurality of sections thatare superposed and separated from one another by one or moreintermediate plates 113, and standing on the bottom plate 111 which doesnot have a central opening so as to close the stack.

[0054] At its top end, the stack is surmounted by a cover 122 providedwith a central opening 122 a in axial alignment with the internal volume121 of the stack.

[0055] Between its inlet into the enclosure 110 through the cover 119and the central opening 122 a, the admitted reagent gas is channeled bya cylindrical wall 118 which may optionally surround a small gaspreheater zone.

[0056] A vertical tubular duct 130 has its top end connected to theopening 122 a and extends down to the plate 111 which closes the bottomend of the duct. The duct 130 may be similar to the duct 30 or the duct40 described above. In the example shown, the duct 130 has a wall 132provided with a plurality of openings 133 that are distributed along thelength and around the axis of the duct.

[0057] The reagent gas admitted into the enclosure is distributed in theinternal volume 121 of the stack of substrates by passing through theopenings 133. The gas passes from the volume 121 to the volume 125outside the stack of substrates by diffusing through the substrates 120and by passing through the passages left between spacers interposedbetween the substrates. The residual gas is extracted from the enclosurethrough the central opening 117 a in the bottom 117 of the enclosure.

[0058] Otherwise, the installation is similar to that of FIG. 1.

[0059] The method and the installation of the invention can be used fordensifying porous substrates other than brake disk preforms, for examplefor substrates constituting preforms 220 for the diverging portions ofrocket engines, as shown in FIG. 8.

[0060] A plurality of substrates 220 are disposed in the same loadingzone of an enclosure 210 with their axial passages in verticalalignment. The bottom substrate is carried by a plate 211 which standson legs 212 a, while the other substrates stand on annular intermediateplates 213. The plates 213 are supported by the support plate 211 viacolumns or posts 212 b.

[0061] With the central openings 213 a in the plates 213 the internalvolumes of the substrates 220 form the internal volume 221 of the stackof substrates. The volume 221 is closed by a cover 222 at its top end.Spacers 223 are interposed between the axial ends of the substrates 220and the plates 211, 213, thereby enabling passages to be left to put thevolume 221 into communication with the volume 225 outside the substratesand inside the enclosure.

[0062] A duct 230 for preheating and distributing the reagent gas isconnected at a bottom end to a central opening 211 a of the plate 211.The duct 230 extends vertically inside the volume 221 to the immediatevicinity of the top of the stack of substrates, where the duct 230 isclosed by a cover 231.

[0063] The side wall 232 of the duct 230 has openings 233, e.g. in theform of perforations, the duct 230 being of the same type as the duct 30in the embodiment of FIGS. 1 and 2.

[0064] Otherwise, the installation is identical to the embodiment ofFIGS. 1 and 2.

[0065] The field of application of the invention is not limited todensifying substrates of annular shape or of hollow axially symmetricalshape.

[0066] Thus, FIG. 9 shows an enclosure 310 having a bottom support plate311 and a plurality of intermediate support plates 313 in a loading zoneof the enclosure 310. The plates 311 and 313 are provided withrespective central openings 311 a and 313 a that are in alignment withan inlet for admitting reagent gas into the enclosure.

[0067] A vertical duct 330 for preheating and distributing the reagentgas has its bottom end connected to the opening 311 a and extendsvertically through the loading zone of the enclosure 310, passingthrough the openings 313 a. At its top end situated in the vicinity ofthe top of the loading zone, the duct 330 is closed by a cover 331.

[0068] The plates 311 and 313 are supported by legs 312 a and by columns312 b.

[0069] The plates 311, 313 support substrates for densifying 320 (notall of them are shown) which may be in a variety of shapes and sizes.

[0070] Otherwise, the installation is identical to that shown in FIGS. 1and 2.

[0071] It should be observed that the method and the installation of theinvention can be implemented for densifying porous substrates withmatrices other than matrices of pyrolytic carbon, for example withceramic matrices. Chemical vapor infiltration processes for ceramicmatrices, e.g. made of silicon carbide (SiC), are well known. Thecomposition of the reagent gas is selected as a function of the natureof the matrix that is to be deposited.

[0072] It should also be observed that the flow section offered by theopenings passing through the side walls of the duct for preheating anddistributing the gas may be distributed uniformly or otherwise along theheight of the duct. A non-uniform distribution may be adoptedspecifically when the need for reagent gas is greater at certain levelsof the tube than at other levels. This can be the case when theconfiguration of the load of substrates and/or the dimensions of thesubstrates vary along the height of the loading zone.

1. A method of densifying porous substrates by chemical vaporinfiltration, the method comprising loading porous substrates fordensification in a loading zone of an enclosure, heating the internalvolume of the enclosure, introducing a reagent gas into the enclosurevia an inlet situated at one end thereof, and preheating the reagent gasafter it has entered into the enclosure and before it comes into contactwith the substrates situated in the loading zone, the method beingcharacterized in that the reagent gas admitted into the enclosure ispreheated, at least in part, by passing along a duct connected to thegas inlet and extending through the loading zone, the duct being raisedto the temperature inside the enclosure, and the preheated reagent gasis distributed into the loading zone through one or more openings formedin the side wall of the duct, along the length thereof.
 2. A methodaccording to claim 1, characterized in that the reagent gas isdistributed through one or more slots extending longitudinally throughthe side wall of the duct.
 3. A method according to claim 1,characterized in that the reagent gas is distributed through a pluralityof perforations formed through the side wall of the duct.
 4. A methodaccording to claim 1, characterized in that the reagent gas flows alongthe duct in contact with walls forming heat exchange surfaces andextending inside the duct.
 5. A method according to claim 1 fordensifying annular substrates disposed in the loading zone as at leastone vertical stack, the method being characterized in that the reagentgas admitted into the enclosure is preheated and distributed inside theenclosure by passing along a duct extending vertically inside the stack.6. A method according to claim 5, characterized in that the reagent gasis distributed solely via openings formed in the side wall of the duct.7. An installation for densifying porous substrates by chemical vaporinfiltration, the installation comprising an enclosure inside whichthere is a zone for loading substrates to be densified, a susceptordefining the enclosure and associated with means for heating theenclosure, a reagent gas inlet at one end of the enclosure, and meanssituated inside the enclosure for preheating the reagent gas, theinstallation being characterized in that a duct is connected to thereagent gas inlet inside the enclosure and extends through the loadingzone, the duct being provided along its length with lateral openingswhich open out into the loading zone in order to distribute the reagentgas therein.
 8. An installation according to claim 7, characterized inthat the openings are in the form of at least one longitudinal slot. 9.An installation according to claim 8, characterized in that the wall ofthe tube is formed by a plurality of panels leaving longitudinal gapsbetween one another.
 10. An installation according to claim 7,characterized in that the openings are in the form of perforationsdistributed along the duct.
 11. An installation according to claim 8,characterized in that the walls are disposed inside the duct.
 12. Aninstallation according to claim 11, characterized in that said internalwalls are in the form of longitudinal panels leaving spaces between oneanother.
 13. A method according to claim 2, characterized in that thereagent gas flows along the duct in contact with walls forming heatexchange surfaces and extending inside the duct.
 14. A method accordingto claim 3, characterized in that the reagent gas flows along the ductin contact with walls forming heat exchange surfaces and extendinginside the duct.
 15. A method according to claim 2 for densifyingannular substrates disposed in the loading zone as at least one verticalstack, the method being characterized in that the reagent gas admittedinto the enclosure is preheated and distributed inside the enclosure bypassing along a duct extending vertically inside the stack.
 16. A methodaccording to claim 3 for densifying annular substrates disposed in theloading zone as at least one vertical stack, the method beingcharacterized in that the reagent gas admitted into the enclosure ispreheated and distributed inside the enclosure by passing along a ductextending vertically inside the stack.
 17. A method according to claim 4for densifying annular substrates disposed in the loading zone as atleast one vertical stack, the method being characterized in that thereagent gas admitted into the enclosure is preheated and distributedinside the enclosure by passing along a duct extending vertically insidethe stack.
 18. A method according to claim 14 for densifying annularsubstrates disposed in the loading zone as at least one vertical stack,the method being characterized in that the reagent gas admitted into theenclosure is preheated and distributed inside the enclosure by passingalong a duct extending vertically inside the stack.
 19. An installationaccording to claim 9, characterized in that the walls are disposedinside the duct.
 20. An installation according to claim 10,characterized in that the walls are disposed inside the duct.
 21. Aninstallation according to claim 19, characterized in that said internalwalls are in the form of longitudinal panels leaving spaces between oneanother.